Transmission between a sensor and a controller in a wireless sensor network

ABSTRACT

Provides: methods for transmission of signals between a sensor and a controller in a wireless sensor network; methods for operating the controller in the wireless sensor network; and methods for transmitting a sensor signal in the wireless sensor network from the sensor to the controller. A wireless sensor network includes a controller connected with multiple antennas for sending out a beacon signal at different instants into different directions and for receiving a sensor signal. Furthermore, the wireless sensor network comprises a sensor having a receiver connected with a sensor antenna for receiving the beacon signal, a transmitter connected with the sensor antenna for sending out the sensor signal, and a control unit which takes care that the sensor signal is transmitted after the beacon signal has been received.

TECHNICAL FIELD

The present invention relates to a wireless sensor network, a wirelesscontroller, a wireless sensor, a method for transmission of signalsbetween the sensor and the controller in the wireless sensor network, amethod for operating the controller in the wireless sensor network, amethod for transmitting a sensor signal in the wireless sensor networkfrom the sensor to the controller and computer program elements forperforming the steps according to the methods.

BACKGROUND OF THE INVENTION

In general, a wireless sensor network (WSN) consists of a set of sensornodes S1, S2, . . . SK which are centrally controlled by means of acentral controller C, which in the following is also referred to as WSNcontroller. The sensor nodes S1, S2, . . . SK are deployed over ageographical area. Each sensor or sensor node S1, S2, . . . SK bymeasurement collects information about a physical phenomenon andforwards its measurement result via a wireless link to the WSNcontroller C for further processing. Since the costs of the sensor nodesS1, S2, . . . SK should be reduced and the sensor nodes are usuallypowered by batteries, the sensor design should aim to reduce theimplementation complexity and the power consumption. The centralcontroller C, however, does not have the same limitations on cost andpower. Therefore, in order to enhance the performance of a WSN, methodsand functions should be identified that can mainly be implemented at thecentral controller C and avoid increasing the complexity of sensor nodesS1, S2, . . . SK.

R. Laroia, P. Visawanath, and D. Tse have described in “Opportunisticbeamforming using dumb antennas”, IEEE Trans. Inform. Theory, vol. 48,pp. 1277-1294, 2002, the concept of opportunistic beamforming with dumbantennas for the downlink in a cellular mobile communication system forexploiting multi-user diversity and suppressing adjacent cellinterference. The scheme uses multiple antennas at the basestation totransmit the same signal from each antenna modulated by a complex gainwhose value is changing in a controlled, but random fashion. Each mobileuser measures the signal-to-interference-plus-noise ratio (SINR) bytracking a pilot signal that is repeatedly radiated at the transmitantennas of the basestation. All mobile users feed back theirmeasurements to the basestation which analyzes the fed-back SINRs valuesin order to form a rule for efficiently scheduling a downlink messagetransfer to the mobile users.

SUMMARY OF THE INVENTION

Therefore, a general aspect of the invention is to provide a wirelesssensor network, a wireless controller, a wireless sensor, a method forwireless data transfer between a sensor and a controller in the wirelesssensor network, and a method for operating the controller wherein thebattery energy consumption in the sensor is reduced. Moreover, thenetwork and method according to the invention can be implemented with areduced signaling overhead, i.e. the media access control (MAC) protocoloverhead can be reduced.

According to an embodiment of the invention, the aspect is achieved by awireless sensor network. The wireless sensor network comprises acontroller connected with multiple antennas for sending out a beaconsignal at different instants into different directions and for receivinga sensor signal. Furthermore, the wireless sensor network comprises asensor having a receiver connected with a sensor antenna for receivingthe beacon signal, a transmitter connected with the sensor antenna forsending out the sensor signal, and a control unit which takes care thatthe sensor signal is transmitted after the beacon signal has beenreceived.

According to another aspect of the invention, a wireless controller forone or more wireless sensors comprises a transmitter connected withmultiple antennas for broadcasting a beacon signal in a determineddirection when the controller is in broadcast mode, and a receiverconnected with the multiple antennas for receiving a sensor signal fromthis determined direction from one of the wireless sensors when thecontroller is in receive mode. The wireless controller further comprisesa signal generator formed such that it takes care that the transmitterbroadcasts the beacon signal in another direction, when the controlleris switched again to broadcast mode. The wireless controller can switchback and forth between receive mode and broadcast mode.

According to another aspect of the invention, the invention provides amethod for operating a controller in a wireless sensor network havingone or more sensors. According to still another aspect of the invention,the invention provides a method for transmitting a sensor signal in awireless sensor network from a sensor to a controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its embodiments will be more fully appreciated byreference to the following detailed description of illustrativeembodiments in accordance with the present invention when taken inconjunction with the accompanying drawings, in which:

FIG. 1 shows a model of a wireless sensor network according to theinvention,

FIG. 2 shows time phases of a MAC protocol, which is implemented partlyin the sensors and partly in the controller of the wireless sensornetwork,

FIG. 3 shows a system model of the wireless sensor network in moredetail,

FIG. 4 shows a block diagram depicting the architecture of the wirelesscontroller with multiple antennas,

FIG. 5 shows a flow diagram of the protocol running on the wirelesscontroller,

FIG. 6 shows a block diagram depicting the architecture of one of thewireless sensors, and

FIG. 7 shows a flow diagram of the protocol running on the wirelesssensors.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a wireless sensor network, a wirelesscontroller, a wireless sensor, a method for wireless data transferbetween a sensor and a controller in the wireless sensor network, and amethod for operating the controller wherein the battery energyconsumption in the sensor is reduced. Moreover, the network and methodaccording to the invention can be implemented with a reduced signalingoverhead, i.e. the media access control (MAC) protocol overhead can bereduced.

In an example embodiment, the invention provides a wireless sensornetwork with the features described below. The wireless sensor networkincludes a controller connected with multiple antennas for sending out abeacon signal at different instants into different directions and forreceiving a sensor signal. The wireless sensor network further comprisesa sensor having a receiver connected with a sensor antenna for receivingthe beacon signal, a transmitter connected with the sensor antenna forsending out the sensor signal, and a control unit which takes care thatthe sensor signal is transmitted after the beacon signal has beenreceived.

In another example embodiment, the invention provides a wirelesscontroller for one or more wireless sensors. The wireless controller forone or more wireless sensors comprises a transmitter connected withmultiple antennas for broadcasting a beacon signal in a determineddirection, when the controller is in broadcast mode, and a receiverconnected with the multiple antennas for receiving a sensor signal fromthis determined direction from one of the wireless sensors, when thecontroller is in receive mode. The wireless controller further comprisesa signal generator formed such that it takes care that the transmitterbroadcasts the beacon signal in another direction, when the controlleris switched again to broadcast mode. The wireless controller can switchback and forth between receive mode and broadcast mode.

In another example embodiment, the invention provides a wireless sensorincluding a receiver connected with a sensor antenna for receiving abeacon signal from a wireless controller, a detecting unit for detectingthe beacon signal, a sensing unit for sensing a physical quantity andgenerating therefrom a sensor signal, and a transmitter connected to thesensor antenna for sending the sensor signal to the wireless controller.

In another example embodiment, the invention provides a method forsignal transmission between a sensor and a controller in a wirelesssensor network. The method for transmission between a sensor and acontroller in a wireless sensor network comprises the following steps.First, the controller broadcasts a beacon signal in a determineddirection and the sensor listens whether the beacon signal is availableand if this is the case the sensor sends the sensor signal to thecontroller. After the controller has broadcasted the beacon signal itlistens whether a sensor signal is available from this direction, and ifthis is the case the controller receives the sensor signal. Afterwardsthe controller broadcasts the beacon signal in another direction, andthen listens again whether a sensor signal is available.

In another example embodiment, the invention provides a method foroperating a controller in a wireless sensor network having one or moresensors. The method for operating a controller in a wireless sensornetwork having one or more sensors comprises the following steps. Abeacon signal is broadcasted in a determined direction by means of thecontroller, and afterwards the controller is listening whether a sensorsignal from one of the sensors can be received from this direction, andif this is the case it receives the sensor signal. Finally, thecontroller broadcasts the beacon signal in another direction and listensagain whether a sensor signal from one of the sensors can be received.

In another example embodiment, the invention provides a method fortransmitting a sensor signal in a wireless sensor network from a sensorto a controller with the features of the independent claim 12. Themethod for transmitting a sensor signal in a wireless sensor networkfrom a sensor to a controller comprises the following steps. The sensoris listening whether a beacon signal from the controller is available,and if this is the case the sensor sends the sensor signal to thecontroller.

In another example embodiment, the invention provides a computer programelement comprising computer program code for performing steps accordingto the above mentioned method when loaded in a digital processor of thecontroller. Finally, the aspect is also achieved by a computer programelement comprising computer program code for performing steps accordingto the above mentioned method when loaded in a digital processor of thesensor.

In an embodiment of the wireless sensor network according to theinvention the controller comprises a random generator which takes carethat the direction into which the beacon signal is send out is randomlyselected. In a further embodiment of the wireless sensor networkaccording to the invention the sensor comprises a detecting unit fordetecting the beacon signal. In a still further embodiment of thewireless sensor network according to the invention the beacon signalcomprises an end-of-beacon symbol.

In the wireless controller according to the invention the signalgenerator can be formed such that the directions into which the beaconsignal is broadcasted are determined randomly. The wireless sensoraccording to the invention can also comprise a sensor control unit whichis formed such that it takes care that the sensor signal is transmittedafter an end-of-beacon symbol in the beacon signal is detected.

It is advantageous in the method for transmission according to theinvention to transmit the sensor signal after an end-of-beacon symbol inthe beacon signal is detected from the sensor. In an embodiment of themethod for transmitting data according to the invention the sensorstarts sending the sensor signal to the controller when the sensor hasdetected the end of the beacon symbol. In another embodiment of themethod for transmitting the sensor signal according to the invention thesensor stays in an energy save mode, also referred to as sleep mode,until the sensor signal is transmitted to the controller.Advantageously, in the method for transmitting the sensor signalaccording to the invention the beacon signal is considered to beavailable if a channel quality indicator of the received signal exceedsa certain threshold value. In an advantageous application of the methodfor transmitting the sensor signal according to the invention thesignal-to-noise-ratio is used as channel quality indicator. Additionalobjects and advantages of the invention will be set forth in thedescription which follows, and may be learned by practice of theinvention.

Hereinafter, an opportunistic multiple-access scheme for controlling theuplink message transfer from sensor nodes S1, S2, . . . SK to a centralcontroller C in a wireless sensor network as shown in FIG. 3 isdescribed. By equipping the controller C with multiple antennas CA1, . .. CAM to communicate with preferably single-antenna sensors S1, S2, . .. SK, a scheme is specified that benefits from diversity reception atthe controller C and leads to a reduction of power consumption in sensornodes S1, S2, . . . SK. This automatically translates into a longerlife-time of sensor batteries. The benefit for the sensor nodes S1, S2,. . . SK stems from diversity reception at the controller C, e.g., fromsimultaneously receiving and processing signals that are received bymultiple antennas CA1, . . . CAM, wherein the processing of the signalsis described further below. The channel coefficients h_(1,1), . . .h_(M,K) and the weights w₁ . . . w_(M) will also be described furtherbelow. Moreover, the proposed technique can be implemented with areduced signaling or protocol overhead. The performance of the scheme isassessed using a Rayleigh fading channel model.

The basic concept of the described scheme is illustrated by means ofFIG. 1. It shows a sensor deployment area comprising numerous sensors,e.g., S1, S2, S3 and a central WSN controller C equipped with multipleantennas. The controller C, also referred to as control node, can form abeam that sweeps in an iterative, pseudo-random fashion across the wholedeployment area. The beam is hence a beacon signal that is sent out atdifferent instants into different directions. Beam forming can beperformed by weighting the signals radiated at each antenna of thecontroller C with complex weights specifying a distinct phase oramplitude shift in the antenna spectrum. The sensors S1, S2, S3, alsoreferred to as sensor nodes, under the beam will receive the beaconsignal from the controller C when it works in downlink-broadcast modewith a larger signal-to-noise ratio (SNR) than those sensors S1, S2, S3which are not under the beam. Since the channel reciprocity principle isvalid for a wireless channel, the same statement is also true for theuplink-transmission mode. Therefore, the sensor nodes S1, S2, S3 thattransmit while covered by the beam will achieve better receive SNRs thanother sensors S1, S2, S3, at the same energy consumption. The powerconsumption in the sensor nodes S1, S2, S3 can thus be reduced byscheduling the message transfer in the uplink-transmission mode if thesensor S1, S2, S3 is covered by the beam. Due to the validity of thereciprocity principle, the sensor nodes S1, S2, S3 can detect beamcoverage by continuously measuring the received SNR or any other channelquality indicator (CQI) of a beacon signal that is repeatedlytransmitted by the controller C. By comparing this measurement to athreshold that can be set, for example, based on previously monitoredand averaged SNR values, the sensors S1, S2, S3 can determineindependently when it is opportune to send a message, also referred toas sensor signal, to the controller C at a reduced signal power. Thescheme is termed as opportunistic because the sensors S1, S2, S3 underthe beam will take advantage of sufficient channel conditions as theyarise, wherein the channel conditions are considered to be sufficient ifthe channel coefficients h are sufficiently close to the weights wdescribed further below in equation 13. A sensor S1, S2, S3 does notrequire a SNR measurement made by any other sensor S1, S2, S3 or thecontroller C to make the decision for initializing a message transfer.

In the following more details on the proposed multiple access scheme aregiven. In section “Opportunistic Multiple Access Scheme”, the techniqueis described from the point of view of a medium-access control (MAC)protocol. Section “Theoretical Foundation” states the theoreticalarguments on which this technique is based.

Opportunistic Multiple Access Scheme

The provided opportunistic multiple access scheme includes somefunctions in the MAC protocol that is implemented partly in the sensorsand partly in the WSN controller. The MAC finite state machine (FSM) 12implemented in the WSN controller C switches repeatedly, but notnecessarily periodically between the two states or phasesDOWNLINK-BROADCAST and UPLINK-TRANSMISSION (see FIG. 2). TheDOWNLINK-BROADCAST phase should be short, but long enough to allowestimation of the SNR at the radio receiver 20 in the sensor S1 (FIG.6). To ensure an efficient throughput, the UPLINK-TRANSMISSION phaseshould be longer; however, the length of this UPLINK-TRANSMISSION phaseis also limited by the assumption that fading effects should notsubstantially change the channel characteristic during a single combinedDOWNLINK-BROADCAST/UPLINK-TRANSMISSION phase.

In the following the expression transceiver is used for a combination oftransmitter and receiver. In FIG. 4 the architecture of the wirelesscontroller C with multiple antennas CA1 . . . CAM is shown. The FSM 12comprises a timer 13 for controlling a first time period T1 and a secondtime period T2. Furthermore, a random number generator 11 is present inthe controller C. The random number generator 11 is also referred to asrandom generator or signal generator and is responsible for generatingrandomly the weights w1, . . . wM. Using multipliers 14.1, 14.2 . . .14.M, the signal posteriority transmitted on each antenna CA1, . . . CAMwill be multiplied by the corresponding weight or complex coefficientw1, . . . wM. This operation is performed upon the request SetBeam.req53 of the FSM 12. The controller C also comprises a radio transceiver 10for receiving or transmitting signals via the antennas CA1 . . . CAM.The FSM 12 can request the radio transceiver 10 to transmit the beaconsignal by issuing the request TxBeacon.req 54. The radio transceiver 10indicates the FSM 12 whether a message has been received by theindication RxMsg.ind 58.

The behavior of FSM 12 is shown in FIG. 5. The state DOWNLINK-BROADCAST,also referred to as broadcast mode, is entered for the first time whenthe controller C is switched on (see reference sign 50 in FIG. 5) andthe timer 13 has been set to the duration of the first time period T1(see reference sign 51). In the state DOWNLINK-BROADCAST and upon thetimeout 52 of the timer 13, the FSM 12 issues a request 53 to the randomnumber generator 11, which randomly chooses a new antenna beam bydrawing a set of random complex coefficients w1, w2 . . . wM andconfiguring the radio transceiver 10 so that the signals to be radiatedat each antenna CA1, CA2, . . . CAM are weighted with a correspondingcomplex coefficient w1, w2 . . . wM via the multipliers 14.1, 14.2, . .. 14.M. After the beam has been formed, the FSM 12 issues a request 54to the radio transceiver 10. Then, the radio transceiver 10 broadcastsinto the determined direction of the beam a beacon signal. The beaconcarries symbols for identifying the channel quality and detecting“end-of-beacon” at the radio transceiver 20 of a sensor S1. After thebeacon signal has been sent and the timer 13 has been set to theduration of the second time period T2 (see reference sign 55), the FSM12 transits to a state UPLINK-TRANSMISSION, also referred to as receivemode. Hence, the FSM 12 switches from the broadcast mode to the receivemode. Without changing the beam forming of the antennas CA1, CA2, . . .CAM, the transceiver 10 starts listening to the radio channel, waitingfor a sensor message Msg. If a sensor message Msg has been detected andreceived within the predefined timespan T2, the radio transceiver 10informs the FSM 12 by issuing the indication RxMsg.ind 58. The FSM 12 ofthe controller C has two options 59: either it returns to stateDOWNLINK-BROADCAST after resetting the timer T2 (see reference sign 60)and setting the timer corresponding to the first time period T1 (seereference sign 57), or it stays in state UPLINK-TRANSMISSION until thetimeout of the timer occurs (see reference sign 56). If no sensormessage Msg has been detected, the expiration of the timer in step 56leads to a transition from the UPLINK-TRANSMISSION phase to theDOWNLINK-BROADCAST phase after setting the timer to the first timeperiod T1 (see reference sign 57). Hence, the FSM 12 switches from thereceive mode to the broadcast mode.

In the following the architecture and functional principle of thewireless sensor nodes S1 to SK is described with the sensor S1, but thedescription is analogously also valid for each of the sensors S2 to SK.In FIG. 6 the architecture of the sensor node S1 is depicted. The FSM 24comprises a timer 25 for controlling a time period T2. The sensor nodeS1 comprises a message source unit 26, also referred to as sensing unit,which measures a physical quantity, therefrom produces the sensormessage Msg to be transmitted and notifies the FSM 24 by issuing thenotification TxMsg.req 71 that a sensor message Msg is pending. The FSM24 issues a request ActivateRx.req 72 to activate the receiver of theradio transceiver 20 which, by using its antenna SA1, senses the mediumfor an incoming beacon. Upon reception of a beacon signal, the radiotransceiver 20 informs the FSM 24 with a message RxBeacon.ind 73. TheFSM 24 issues a request ActivateTx.req 77 to activate the transmitter ofthe radio transceiver 20. The radio transceiver 20 passes the beaconsignal to a CQI computation unit 21 and to a THR computation unit 22.Those units 21 and 22 will respectively produce the metrics CQI and THR,which will be passed to a comparator 23. The units 21, 22 and 23 arehereinafter also called detecting unit. Depending on the output (yes orno) of the comparator 23, the FSM 24 will issue a transmission requestTxMsg.req 79 to the transmitter of the radio transceiver 20.

As depicted in FIGS. 6 and 7 the FSM 24 implemented in the wirelesssensor S1 comprises the states SLEEP, RECEIVE, and TRANSMIT. When thesensor S1 is switched on (see reference sign 70), the sensor node S1 isin the sleep mode SLEEP, also referred to as energy save mode, that is,the radio transceiver 20 is powered down in order to reduce dissipationof battery energy. When a message transfer request TxMsg.req 71 has beenraised, the FSM 24 activates the receiver of the radio transceiver 20 byissuing a request ActivateRx.req 72 and enters the state RECEIVE. Thesensor S1 listens to the radio channel and waits for receiving apredefined, beacon signal transmitted by the controller C. If a beaconsignal has been recognized, the radio transceiver 20 informs the FSM24by issuing an indication RxBeacon.ind(T2) 73. The timer 25 is set to thesecond time period T2 74 as passed by the beacon signal in step 73. Thenin step 75 the metric CQI, e.g. the SNR, is computed and in step 76compared to a decision threshold THR. If the metric CQI is not largerthan the decision threshold THR a NO is issued at the output of thecomparator 23, the FSM 24 returns to state RECEIVE. In case thecomparator 23 issues a YES at its output, the sensor S1 transits tostate TRANSMIT after activating the transmitter of the radio transceiver20 with message ActivateTx.req 77. After the timeout 78, the FSM 24issues the request TxMsg.req 79 to the radio transceiver 20 to transmitthe sensor message Msg. After the sensor message Msg has been sent, theFSM 24 of the sensor S1 returns to state SLEEP.

The decision rule, or more specifically the decision threshold THR to beset in the MAC FSM 24 of the sensor S1, depends on the statistics of thewireless channel. If the channel can be modeled by the Rayleighslow-fading channel model, a Maximum Likelihood (ML) rule is proposed tobe implemented to decide whether the sensor node S1 should transmit ornot. In Section “Theoretical Foundation”, it will be shown that,firstly, the decision to transmit only under sufficient channelconditions can be based on the estimated SNR and, secondly, how toderive the decision threshold THR for this particular statisticalchannel model. Other suboptimal, but more practical strategies to setthe threshold THR are conceivable. For example, the SNR values monitoredduring previous broadcast phases can be collected to form a long-termSNR statistic in the sensor node S1. Based on this statistic, thedecision threshold THR can be set so that the sensor node S1 onlytransmits if the current SNR value is above the long-term average SNRvalue.

In the proposed multiple access scheme, the number of available,randomly selectable antenna beams formed by the controller C ispreferably larger than the number of sensors S1, S2, . . . SK in theWSN. This reduces the likelihood that various sensor nodes will monitorsimilar channel properties and thus simultaneously decide for favorablechannel conditions, which would lead to simultaneous message transfersand collisions. If the number of beams is limited to a smaller number,the proposed scheme can be combined with well-known conventionalmultiple access techniques to coordinate the simultaneous transmissionof sensor nodes seeing a channel with sufficient channel conditions. Forinstance, a simplified carrier-sense mechanism with random backoff mightbe used in the UPLINK TRANSMISSION phase to reduce the number ofcollisions, and the exchange of RTS/CTS frames between sensor S1 andcontrol node C before sensor message transmission in order to combat thehidden terminal problem. The RTS/CTS mechanism is used to reduceinterruptions from other stations during a transmission from a source toa destination. The source sends a Request To Send (RTS) to thedestination. The latter answers with a Clear To Send (CTS). RTS(respectively CTS) notifies the stations in the neighborhood of thesource (respectively in the neighborhood of the destination) that atransmission will take place and that they should delay their owntransmission during the amount of time indicated in the RTS(respectively CTS). Other solutions that allow simultaneous transmissionof multiple nodes, e.g. Code Division Multiple Access, might also beenvisaged.

The basic scheme presented above might be extended by applying somewell-known wireless protocol and transceiver concepts. For example, athird phase might follow the UPLINK TRANSMISSION phase in order toacknowledge the successful message reception. Additionally, the WSNcontroller C could perform channel estimation on the UPLINK TRANSMISSIONand use this information to adapt the drawing statistics of the antennacoefficients w1, w2, . . . wM accordingly.

Theoretical Foundation

This section describes the mathematical foundation of the proposedmultiple-access technique. First, the system model will be introduced,then a formal description of the operations taking place during theDOWNLINK-BROADCAST phase is given, and finally the decision thresholdTHR for the statistics of a particular radio channel model will bederived that is also applied for the design of radio receivers andinvestigations of channel fading effects on the receiver performance.

System Model

An illustration of the system model is depicted in FIG. 3. The basebandrepresentation of the signal received from sensor node k at the WSNcontroller C is given by:i·y _(k)=√{square root over (ε_(s))}h _(k) s _(k) +n _(k),  (1)

where k=1, . . . K,

where the above used sensor indicators S1, . . . SK are replaced by 1, .. . K, and

-   -   where the vectors y_(k), h_(k), and n_(k) are given by:        $\begin{matrix}        {{y_{k} = \begin{bmatrix}        y_{k,1} \\        y_{k,2} \\        M \\        y_{k,M}        \end{bmatrix}},{h_{k} = \begin{bmatrix}        h_{k,1} \\        h_{k,2} \\        M \\        h_{k,M}        \end{bmatrix}},{{{and}\quad n_{k}} = \begin{bmatrix}        n_{k,1} \\        n_{k,2} \\        M \\        n_{k,M}        \end{bmatrix}}} & (2)        \end{matrix}$

The scalars Y_(k,m) where m=1, . . . M denote the sensor signal sent bythe kth sensor node k and received by the controller C on the mthantenna CAm. The indices in the channel coefficients h_(k,m) where m=1,. . . M and in the noise terms n_(k,m) where m=1, . . . M can beinterpreted in a similar fashion. The transmitted energy is denoted byε_(s), and S_(k) is the transmitted symbol drawn from an unit-energymodulation constellation. The noise is assumed to be white and complexGaussian distributed, i.e. n_(k,m)˜CN(0,N₀) andε{n_(k,m)n_(k,n)*}=N₀δ(m−n). The channel coefficients h_(k,m) can bemodeled in several ways depending on the radio propagation environment.In order to derive the decision threshold THR, a Rayleigh slow-fadingchannel model is assumed. The channel coefficients h_(k,m) are thusmodeled as independent complex Gaussian coefficients, i.e. CN (0,1), andthe coherence time of the channel is assumed to be large enough toconsider the channel characteristics of the down- and uplink as equal.Under these assumptions, the magnitude squared of the channelcoefficients follows a chi-square distribution. The cumulativedistribution function (CDF) and the probability density function (PDF)of a X_(2m) ² random variable are given by: $\begin{matrix}{{F_{x_{2m}^{2}}(x)} = {1 - {{\mathbb{e}}^{- x}{\sum\limits_{k = 0}^{m - 1}\quad\frac{x^{k}}{k!}}}}} & (3) \\{{f_{x_{2m}^{2}}(x)} = {{\frac{\partial}{\partial x}{F_{x_{2m}^{2}}(x)}} = {{\frac{{\mathbb{e}}^{- x}x^{m - 1}}{\left( {m - 1} \right)!}\quad{for}\quad x} \geq 0}}} & (4)\end{matrix}$

Upon reception of the kth sensor signal y_(k), the controller C weighsthe output signal from all antennas CA1, . . . CAM and adds them up.This operation can be expressed by the following inner product:r _(k) =w _(k) ^(H) y _(k)=√{square root over (ε_(s))}·w _(k) ^(H) h_(k) s _(k) +w _(k) ^(H) n _(k)  (5)

where W_(k) ^(H)=weight vector.

The output r_(k) of this combiner is a scalar. In order to increase thepost-processing SNR, the weights are chosen such that w_(k)=h_(k). Asufficient statistic is then obtained as:r _(k)=√{square root over (ε_(s))}·∥h _(k)∥² s _(k) +h _(k) ^(H) n_(k)  (6)

This weighting is known as Maximum Ratio Combining (MRC) and yields thereceived SNR $\rho_{k} = {\frac{ɛ_{s}}{N_{0}} \cdot {{h_{k}}^{2}.}}$

Normally, in this technique the receiving sensor uses precise channelknowledge. The resulting overhead for channel estimation is usuallyprohibitive in a wireless sensor network. However, the proposedopportunistic scheme according to the invention achieves under someconstraints, which are explained in the following, the MRC performanceon the uplink without any channel knowledge.

The Downlink Broadcast Phase

During the DOWNLINK-BROADCAST phase, the WSN controller C forms theantenna beam by choosing the weights w randomly according to:$\begin{matrix}{w = \sqrt{\frac{M}{{g}^{2}}g}} & (7)\end{matrix}$

where g_(m)˜h_(k,m), ∀_(k), m=1, . . . M, and

where g_(m)˜h_(k,m) means that g_(m) and h_(k,m) are identicallydistributed.

The coefficients g_(m), m=1, . . . M are identically distributed as thechannel coefficients, and the normalization term ensures that the powerconstraint w^(H)w=M is satisfied. The weights w are chosen independentlyof a particular sensor node. Once the transmission weights w are chosen,the controller C uses them to broadcast the beacon signal. At the nextDOWNLINK-BROADCAST phase, new antenna coefficients w1, . . . wM will bedrawn.

Because it is a continuous distribution, a practical scheme quantizesthe distribution in equation (7). Let L denote the number ofquantization levels. Due to the vectorial character of the distribution,the weight distribution is quantized on an antenna basis. Thus, thetotal number of beams to be visited is N=L^(M). The beams are visitedrandomly according to a discrete probability distribution function.

If the weights match the channel of sensor node k in the MRC sense, theSNR ρ_(k) ^(B) at the sensor node covered by the beam will be:$\begin{matrix}{\rho_{k}^{B} = {{\frac{ɛ_{s}}{N_{0}}{\sum\limits_{m = 1}^{M}\quad{h_{k,m}}^{2}}} = {\frac{ɛ_{s}}{N_{0}}{h_{k}}^{2}}}} & (8)\end{matrix}$

If the number of sensor nodes K is high enough, then, with a sufficientprobability, there will be at least one sensor node that matches theweights or is sufficiently close to match them and will receive thesignal with the SNR given by equation (8).

This result has been presented in the above mentioned paper of R. Laroiaet al, where after the broadcast phase, all the mobile users in acellular network feed back to the controller their estimated SNR.According to the set of SNRs, the controller schedules the strongestuser for transmission. Using this scheme, the referred work shows thatasymptotically in the number of users, the transmission to a user isscheduled when the weights w=h_(k), i.e. the user is in beamformingconfiguration. Transmitting at that moment is advantageous in aninformation-theoretic sense. The achievable rates are improved and thegains increase with the number of users.

In contrast thereto in the invention no information is fed back to thecontroller. The sensor nodes determine by themselves whether to transmitor not. In fact, in a probabilistic sense, they estimate how well theyare covered by the controller's antenna beam based on their own SNRmeasurement performed during the downlink broadcasting phase. Therefore,the sensors will transmit in case they conclude to be substantiallyunder the beam.

SNR Statistics

As the decision of whether to transmit or not is done based on the SNRobserved by each sensor node, it is of interest to analyze itsstatistics. Using equation (7), the SNR ρ_(k) of the received beaconsignal in the sensor node can be written as a quadratic form and itsgeneral distribution can then be readily obtained. $\begin{matrix}\begin{matrix}{\rho_{k} = {{\frac{ɛ_{s}}{{MN}_{0}}h_{k}^{H}w} = {\frac{ɛ_{s}}{N_{0}{g}^{2}}\left( {h_{k}^{H}g} \right)^{2}}}} \\{= {\frac{ɛ_{s}}{N_{0}{g}^{2}}h_{k}^{H}{gg}^{H}h_{k}}} \\{= {\frac{ɛ_{s}}{N_{0}{g}^{2}}h_{k}^{H}{U\Lambda}\quad U^{H}h_{k}}} \\{= {\frac{ɛ_{s}}{N_{0}{g}^{2}}{\sum\limits_{m = 0}^{M}\quad{{{Uh}_{k}}_{m}^{2}{\lambda_{m}\left( {gg}^{H} \right)}}}}} \\{= {\frac{ɛ_{s}}{N_{0}}{\left. {{\overset{\sim}{h}}_{k,m}}^{2} \right.\sim\frac{ɛ_{s}}{N_{0}}}x_{2}^{2}}}\end{matrix} & (9)\end{matrix}$

where the eigenvalue decomposition of the rank-deficient matrix gg^(H),the fact that its unique non-zero eigenvalue equals ||g||², and that{tilde over (h)}_(k)=Uh_(k) is equally distributed as h_(k) is used.

On the other hand, the best SNR value is obtained when the transmitweights match the channel coefficients h_(k) as indicated in equation(8). In this case, the SNR distribution is given by $\begin{matrix}{{\left. \rho_{k} \right.\sim\frac{ɛ_{s}}{N_{0}}}x_{2M}^{2}} & (10)\end{matrix}$

The two results on the distribution of the SNR can be used to obtain thefollowing conditional distributions: Let A_(k) (respectively, {overscore(A)}_(k)) denote the event that the sensor k is covered by the antennabeam (respectively, the sensor k is not covered). Then, the distributionof the SNR ρ_(k) conditioned on the event A_(k) is given by:$\begin{matrix}\begin{matrix}{{f\left( {\rho_{k}❘A_{k}} \right)} = {\frac{1}{\rho}{f_{x_{2M}^{2}}\left( {\rho_{k}/\rho} \right)}}} \\{= {\frac{1}{\rho}\frac{{{\mathbb{e}}^{{- \rho}\quad{k/\rho}}\left( {\rho_{k}/\rho} \right)}^{M - 1}}{\left( {M - 1} \right)!}}}\end{matrix} & (11)\end{matrix}$

where ρ denotes the pre-processing SNR ε_(s)/N₀. Similarly, thedistribution of the SNR ρ_(k) conditioned on the complementary event{overscore (A)}_(k) is: $\begin{matrix}\begin{matrix}{{f\left( {\rho_{k}❘{\overset{\_}{A}}_{k}} \right)} = {\frac{1}{\rho}{f_{x_{2}^{2}}\left( {\rho_{k}/\rho} \right)}}} \\{= {\frac{1}{\rho}{\mathbb{e}}^{{- \rho}\quad{k/\rho}}}}\end{matrix} & (12)\end{matrix}$

These two conditional distributions f(ρ_(k)|A_(k)) andf(ρ_(k)|{overscore (A)}_(k)) will now be exploited to derive a rule thatcan be implemented in the sensor to detect whether it is covered by thebeam or not.

To Transmit or Not To Transmit

Hereafter, a method is proposed which allows the sensor to detect basedon the observed SNR ρ_(k) whether the event A_(k) is true or false. Onecan say that A_(k) happens when the weight vector w is such that:$\begin{matrix}{w \in \left\lbrack {{{\sqrt{\frac{M}{{h_{k}}^{2}}}h_{k}} - \gamma},{{\sqrt{\frac{M}{{h_{k}}^{2}}}h_{k}} + \gamma}} \right\rbrack} & (13)\end{matrix}$

where γ is sufficiently small. This γ-approximation is necessary due tothe continuous character of the considered distributions. The Maximum APosteriori (MAP) decision rule is formed as follows: $\begin{matrix}{{\Pr\left( A_{k} \right)}f\text{(}\rho_{k}\left. A_{k} \right)\begin{matrix}\underset{>}{A_{k}} \\\underset{{\overset{\_}{A}}_{k}}{<}\end{matrix}{\Pr\left( {\overset{\_}{A}}_{k} \right)}{f\left( \rho_{k} \right.}{\overset{\_}{A}}_{k}\text{)}} & (14)\end{matrix}$

The Maximum Likelihood rule is obtained for equal a prioriprobabilities. Using equations (11), and (12), and the Chi-squaredensity function equation (3), the decision rule is obtained:$\begin{matrix}\left. {\alpha\frac{\left( {\rho_{k}/\rho} \right)^{M - 1}}{\left( {M - 1} \right)!}\begin{matrix}\underset{>}{A_{k}} \\\underset{{\overset{\_}{A}}_{k}}{<}\end{matrix}1}\Leftrightarrow{\rho_{k}\begin{matrix}\underset{>}{A_{k}} \\\underset{{\overset{\_}{A}}_{k}}{<}\end{matrix}\exp\quad\left( {\frac{{\sum\limits_{i = 1}^{M - 1}\quad{\log\quad i}} - {\log\quad\alpha}}{\left( {M - 1} \right)} + {\log\quad\rho}} \right)} \right. & (15)\end{matrix}$

for detecting the event beam-coverage in the sensor k. The decisionthreshold depends on the number M of antennas CA at the controller C, onthe a priori probabilities of both events contained inα=PR(A_(k))/Pr({overscore (A)}_(k)), and on the pre-processing SNR ρ.

Any disclosed embodiment may be combined with one or several of theother embodiments shown and/or described. This is also possible for oneor more features of the embodiments. Having illustrated and described anadvantageous embodiments for novel methods and apparatus, it is notedthat variations and modifications in the method and the apparatus can bemade without departing from the spirit of the invention or the scope ofthe appended claims.

The present invention can be realized in hardware, software, or acombination of hardware and software. It may be implemented as a methodhaving steps to implement one or more functions of the invention, and/orit may be implemented as an apparatus having components and/or means toimplement one or more steps of a method of the invention described aboveand/or known to those skilled in the art. A visualization tool accordingto the present invention can be realized in a centralized fashion in onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system—or other apparatus adapted for carrying out the methodsand/or functions described herein—is suitable. A typical combination ofhardware and software could be a general purpose computer system with acomputer program that, when being loaded and executed, controls thecomputer system such that it carries out the methods described herein.The present invention can also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which—when loaded in a computersystem—is able to carry out these methods. Methods of this invention maybe implemented by an apparatus which provides the functions carrying outthe steps of the methods. Apparatus and/or systems of this invention maybe implemented by a method that includes steps to produce the functionsof the apparatus and/or systems.

Computer program means or computer program in the present contextinclude any expression, in any language, code or notation, of a set ofinstructions intended to cause a system having an information processingcapability to perform a particular function either directly or afterconversion to another language, code or notation, and/or afterreproduction in a different material form.

Thus the invention includes an article of manufacture which comprises acomputer usable medium having computer readable program code meansembodied therein for causing one or more functions described above. Thecomputer readable program code means in the article of manufacturecomprises computer readable program code means for causing a computer toeffect the steps of a method of this invention. Similarly, the presentinvention may be implemented as a computer program product comprising acomputer usable medium having computer readable program code meansembodied therein for causing a function described above. The computerreadable program code means in the computer program product comprisingcomputer readable program code means for causing a computer to affectone or more functions of this invention. Furthermore, the presentinvention may be implemented as a program storage device readable bymachine, tangibly embodying a program of instructions executable by themachine to perform method steps for causing one or more functions ofthis invention.

It is noted that the foregoing has outlined some of the more pertinentobjects and embodiments of the present invention. This invention may beused for many applications. Thus, although the description is made forparticular arrangements and methods, the intent and concept of theinvention is suitable and applicable to other arrangements andapplications. It will be clear to those skilled in the art thatmodifications to the disclosed embodiments can be effected withoutdeparting from the spirit and scope of the invention. The describedembodiments ought to be construed to be merely illustrative of some ofthe more prominent features and applications of the invention. Otherbeneficial results can be realized by applying the disclosed inventionin a different manner or modifying the invention in ways known to thosefamiliar with the art.

1. A wireless sensor network, comprising: a controller connected withmultiple antennas for sending out a beacon signal at different instantsinto different directions and for receiving a sensor signal, and asensor comprising: a receiver connected with a sensor antenna forreceiving the beacon signal, a transmitter connected with the sensorantenna for sending out the sensor signal, and a control unit for takingcare that the sensor signal is transmitted after the beacon signal hasbeen received.
 2. The wireless sensor network according to claim 1,wherein the controller comprises a random generator for randomlydetermining a direction into which the beacon signal is sent out.
 3. Thewireless sensor network according to claim 1, wherein the sensor furthercomprises a detecting unit for detecting the beacon signal.
 4. Thewireless sensor network according to claim 1, wherein the beacon signalcomprises an end-of-beacon symbol.
 5. A wireless controller for at leastone wireless sensor, comprising: a transmitter connected with multipleantennas for broadcasting a beacon signal in a determined direction,when the controller is in a broadcast mode, a receiver connected withthe multiple antennas for receiving a sensor signal from the determineddirection from one of said at least one wireless sensor, when thewireless controller is in a receive mode, and a signal generatordesigned to take care that the transmitter broadcasts the beacon signalin another direction when the controller is switched again to thebroadcast mode.
 6. The wireless controller according to claim 5, whereinthe signal generator is designed to effect a random determination of thedirections into which the beacon signal is broadcasted.
 7. A wirelesssensor, comprising: a receiver connected with a sensor antenna forreceiving a beacon signal from a wireless controller, a detecting unitfor detecting the beacon signal, a sensing unit for sensing a physicalmagnitude and generating therefrom a sensor signal, and a transmitterconnected with the sensor antenna for sending the sensor signal to thewireless controller.
 8. The wireless sensor according to claim 7,further comprising a sensor control unit which is designed to take carethat the sensor signal is transmitted after an end-of-beacon symbol inthe beacon signal has been detected.
 9. A method for transmission ofsignals between a sensor and a controller in a wireless sensor network,comprising the steps of: a) the controller broadcasting a beacon signalin a determined direction, b) the sensor listening whether the beaconsignal is available and if this is the case sending a sensor signal tothe controller, c) the controller listening whether said sensor signalis available from the determined direction, and if this is the casereceiving the sensor signal, d) the controller broadcasting the beaconsignal in another direction, e) repeating steps b) and c).
 10. Themethod according to claim 9, wherein the sensor signal is transmitted bythe sensor after an end-of-beacon symbol has been detected in the beaconsignal.
 11. A method for operating a controller in a wireless sensornetwork having at least one sensor, comprising the steps of: a)broadcasting a beacon signal in a determined direction by means of thecontroller, b) listening whether a sensor signal from one of the sensorscan be received from the determined direction, and if this is the casereceiving the sensor signal, c) broadcasting the beacon signal inanother direction, and d) repeating steps b) and c).
 12. A method fortransmitting a sensor signal in a wireless sensor network from a sensorto a controller, comprising the steps of: a) listening whether a beaconsignal from the controller is available, and b) if this is the casesending the sensor signal to the controller.
 13. The method according toclaim 12, wherein the sensor starts sending the sensor signal to thecontroller when the sensor has detected an end-of-beacon symbol.
 14. Themethod according to claim 12, wherein the sensor stays in an energy savemode until the sensor signal is transmitted to the controller.
 15. Themethod according to claim 12, wherein the beacon signal is considered tobe available if a channel quality indicator of the received beaconsignal exceeds a predetermined threshold value.
 16. The method accordingto claim 15, wherein the channel quality indicator is selected tocomprise the signal-to-noise-ratio of the beacon signal.
 17. A computerprogram product for operating a controller in a wireless sensor networkhaving at least one sensor, the computer program product comprising: astorage medium readable by a processing circuit and storing instructionsfor execution by the processing circuit for performing a methodcomprising: a) broadcasting a beacon signal in a determined direction bymeans of the controller, b) listening whether a sensor signal from oneof the sensors can be received from the determined direction, and ifthis is the case receiving the sensor signal, c) broadcasting the beaconsignal in another direction, and d) repeating steps b) and c).
 18. Acomputer program product for operating a controller in a wireless sensornetwork having at least one sensor, the computer program productcomprising: a storage medium readable by a processing circuit andstoring instructions for execution by the processing circuit forperforming a method comprising: a) listening whether a beacon signalfrom the controller is available, and b) if this is the case sending thesensor signal to the controller.
 19. A computer program product foroperating a controller in a wireless sensor network having at least onesensor, the computer program product comprising: a storage mediumreadable by a processing circuit and storing instructions for executionby the processing circuit for performing a method comprising: a) thecontroller broadcasting a beacon signal in a determined direction, b)the sensor listening whether the beacon signal is available and if thisis the case sending a sensor signal to the controller, c) the controllerlistening whether said sensor signal is available from the determineddirection, and if this is the case receiving the sensor signal, d) thecontroller broadcasting the beacon signal in another direction, e)repeating steps b) and c).